A well known example of a navigation system is the global positioning system (GPS) satellite-based system which uses a GPS receiver to receive location fixes from the GPS satellites to determine the location of the navigator within a navigation environment. The GPS receiver periodically receives information from broadcasting GPS satellites and uses that information to triangulate the position of the navigator. However, in certain environments, GPS systems suffer from limited availability of the GPS signals and fail to work as well as may be desired. For example, in an urban environment, tall buildings may obstruct or degrade GPS satellite reception. Obstruction or degradation of GPS reception because of limited view of the open sky, or because of signal multipath or reflection, may create a “GPS denied” environment in which the GPS receiver is unable to obtain a location fix from the GPS satellite.
There have been several attempts to address the sporadic loss of a GPS signal; however, the need for accurate absolute positioning in urban environments for transportation systems has few practical yet technically feasible solutions. Some solutions have attempted to use image processing or video analytics “scene matching” to determine the position of a sensor based on an image of the scene taken by the sensor. A drawback to these solutions however, is the need for an extensive database of scenery and enormous computational resources to perform the scene matching processing. The scenery database must also be kept current as seasonal or permanent changes to the physical environment can significantly impact scene matching accuracy. Another solution uses sophisticated “dead reckoning” calculations to locate the navigator when GPS reception is lost or is unavailable. However, such systems suffer significant drawbacks, including displacement and heading errors that accumulate over time (drift or bias), and, in commercially viable vehicle-based systems, an inability to adapt to rapid and frequent changes in speed and direction of the navigator.